The present disclosure is directed to a simultaneous fluid flow and fluid density measurement device. It is a device which is adapted to be suspended in a well borehole, typically a cased well, and in a well which is producing fluid which flows through the well to the surface. The system can be temporarily or permanently installed at a location. If permanent, a communication line from the device is installed which extends to the surface, or the data can be recorded within the device with a "memory gauge," and retrieved when the device is eventually removed from the well borehole. This eliminates the need to install a line to the surface.
In ordinary usage to determine oil and gas production from various zones, the device is lowered to a particular depth in the borehole on a logging cable having one or more electrical conductors therein. In most cases the fluid is flowing upward within the well bore; therefore, the fluid flows upward through the device. For purposes of illustration, assume that a packer is inflated to insure that all fluid flows through the device. The device can be used without a packer if it is assumed that the flow through the device is representative of the total flow within the borehole. The device can even be moved continuously upward within the well bore, so long as the linear velocity of the fluid exceeds the linear velocity of the device. The apparent flow through the device can be converted to actual flow through the device if the linear velocity of the device is measured.
The fluid is directed through an inlet port at the bottom of a tool containing the measuring device, flows upwardly through a vertical tube, and emerges at an outlet port at the top of the tool. The fluid moves a float within an axially positioned tube which slides or traverses the tube between inlet and outlet ports. The flotation or position of the float within the tube is, for a given float mass and cross sectioned area, dependent primarily upon the flow rate and the density of the fluid.
In the preferred embodiment, a radioactive source is installed in the float. One example is isotope 137 of cesium which emits gamma radiation. First and second radioaction responsive gamma ray detectors are included in the tool. They are located at respective ends of the tube. One is at the inlet and the other at the outlet, both supported by the elongated tool body which supports and defines the present structure.
The source-detector technique is used to (1) determine the position of the float within the tool and (2) to determine the density of the fluid. Both of these parameters are required to determine the volume flow rate through the tool. For a fluid of given density, as the flow rate increases, the float is raised further in the tube. The counting rate in the upper detector increases as the float, which contains a radioactive source, moves closer. Conversely, the counting rate in the lower detector decreases as the float moves away. Through the use of the foregoing radioactive source and two detector system, the relative position of the float can be determined from the ratio of detector count rates. For a given flow rate, thus float position, the counting rates recorded from each detector are a function of the density of the fluid. As density increases, counting rate decreases. By calibrating the detectors, fluid density can simultaneously be determined. Knowing float position and fluid density, volume flow rate can be determined.
Flow rates in boreholes are currently measured using technologies such as spinners, vortex flow meters, and pitot meters. The present invention has several advantages over these technologies, in that, it is relatively simple both mechanically and electrically, it is compact and easy to construct, and it has an extremely large dynamic range, especially if the mass and the cross-sectional area of the float are varied by exchanging floats. In addition, fluid density is measured simultaneously with volume flow rate. In oil well production monitoring, fluid density is a key parameter of interest being used to determine relative concentrations of produced water, oil, and gas. In the past, fluid density has been measured with an entirely separate instrument.